[v3] PCI: Add D3 support for PCI bridges in DT based platforms

  Currently, PCI core will enable D3 support for PCI bridges only when the
following conditions are met:

1. Platform is ACPI based
2. Thunderbolt controller is used
3. pcie_port_pm=force passed in cmdline

While options 1 and 2 do not apply to most of the DT based platforms,
option 3 will make the life harder for distro maintainers. Due to this,
runtime PM is also not getting enabled for the bridges.

To fix this, let's make use of the "supports-d3" property [1] in the bridge
DT nodes to enable D3 support for the capable bridges. This will also allow
the capable bridges to support runtime PM, thereby conserving power.

Ideally, D3 support should be enabled by default for the more recent PCI
bridges, but we do not have a sane way to detect them.

[1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
---
This patch is tested on Qcom SM8450 based development board with an out-of-tree
DT patch.

NOTE: I will submit the DT patches adding this property for applicable bridges
in Qcom SoCs separately.

Changes in v3:
- Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
- Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org

Changes in v2:
- Switched to DT based approach as suggested by Lukas.
- Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
---
 drivers/pci/of.c  | 12 ++++++++++++
 drivers/pci/pci.c |  3 +++
 drivers/pci/pci.h |  6 ++++++
 3 files changed, 21 insertions(+)


---
base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
change-id: 20240131-pcie-qcom-bridge-b6802a9770a3

Best regards,
  

On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > Currently, PCI core will enable D3 support for PCI bridges only when the
> > following conditions are met:
> 
> Whenever I read "D3", I first have to figure out whether we're talking
> about D3hot or D3cold.  Please save me the effort :)
> 

Both actually, that's why I used "D3" as in the spec. I should've explicitly
mentioned that in the commit message.

> > 1. Platform is ACPI based
> > 2. Thunderbolt controller is used
> > 3. pcie_port_pm=force passed in cmdline
> 
> Are these joined by "AND" or "OR"?  I guess probably "OR"?
> 
> "... all the following conditions are met" or "... one of the
> following conditions is met" would clarify this.
> 

Will use "one of the..."

> > While options 1 and 2 do not apply to most of the DT based platforms,
> > option 3 will make the life harder for distro maintainers. Due to this,
> > runtime PM is also not getting enabled for the bridges.
> > 
> > To fix this, let's make use of the "supports-d3" property [1] in the bridge
> > DT nodes to enable D3 support for the capable bridges. This will also allow
> > the capable bridges to support runtime PM, thereby conserving power.
> 
> Looks like "supports-d3" was added by
> https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> The commit log mentions "platform specific ways", which suggests maybe
> this is D3cold, since D3hot should be supported via PMCSR without any
> help from the platform.
> 
> So I *guess* this really means "platform provides some non-architected
> way to put devices in D3cold and bring them back to D0"?
> 

By reading the comments and git log of the pci_bridge_d3_possible() function in
drivers/pci/pci.c, we can understand that some of the old bridges do not support
both D3hot and D3cold. And to differentiate such bridges, platforms have to
notify the OS using some ways.

ACPI has its own implementation [1] and DT uses "supports-d3" property.

And yes, in an ideal world PMCSR should be sufficient for D3hot, but you know
the PCI vendors more than me ;)

> > Ideally, D3 support should be enabled by default for the more recent PCI
> > bridges, but we do not have a sane way to detect them.
> > 
> > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31
> 
> This link won't remain accurate as lines are added/removed.  The
> kernel.org cgit allows specific commits
> (https://git.kernel.org/linus/0dd3ee311255) or line references at
> specific commits or tags
> (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/process/submitting-patches.rst?id=v6.0#n94)
> 

I'm not aware of such references in github. So I'll reference the commit
instead.

- Mani

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci-acpi.c#n976

> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > ---
> > This patch is tested on Qcom SM8450 based development board with an out-of-tree
> > DT patch.
> > 
> > NOTE: I will submit the DT patches adding this property for applicable bridges
> > in Qcom SoCs separately.
> > 
> > Changes in v3:
> > - Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
> > - Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org
> > 
> > Changes in v2:
> > - Switched to DT based approach as suggested by Lukas.
> > - Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
> > ---
> >  drivers/pci/of.c  | 12 ++++++++++++
> >  drivers/pci/pci.c |  3 +++
> >  drivers/pci/pci.h |  6 ++++++
> >  3 files changed, 21 insertions(+)
> > 
> > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > index 51e3dd0ea5ab..24b0107802af 100644
> > --- a/drivers/pci/of.c
> > +++ b/drivers/pci/of.c
> > @@ -786,3 +786,15 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> >  	return slot_power_limit_mw;
> >  }
> >  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > +
> > +/**
> > + * of_pci_bridge_d3 - Check if the bridge is supporting D3 states or not
> > + *
> > + * @node: device tree node of the bridge
> > + *
> > + * Return: %true if the bridge is supporting D3 states, %false otherwise.
> > + */
> > +bool of_pci_bridge_d3(struct device_node *node)
> > +{
> > +	return of_property_present(node, "supports-d3");
> > +}
> > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > index d8f11a078924..8678fba092bb 100644
> > --- a/drivers/pci/pci.c
> > +++ b/drivers/pci/pci.c
> > @@ -1142,6 +1142,9 @@ static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> >  	if (pci_use_mid_pm())
> >  		return false;
> >  
> > +	if (dev_of_node(&dev->dev))
> > +		return of_pci_bridge_d3(dev->dev.of_node);
> > +
> >  	return acpi_pci_bridge_d3(dev);
> >  }
> >  
> > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > index 2336a8d1edab..10387461b1fe 100644
> > --- a/drivers/pci/pci.h
> > +++ b/drivers/pci/pci.h
> > @@ -635,6 +635,7 @@ int of_pci_get_max_link_speed(struct device_node *node);
> >  u32 of_pci_get_slot_power_limit(struct device_node *node,
> >  				u8 *slot_power_limit_value,
> >  				u8 *slot_power_limit_scale);
> > +bool of_pci_bridge_d3(struct device_node *node);
> >  int pci_set_of_node(struct pci_dev *dev);
> >  void pci_release_of_node(struct pci_dev *dev);
> >  void pci_set_bus_of_node(struct pci_bus *bus);
> > @@ -673,6 +674,11 @@ of_pci_get_slot_power_limit(struct device_node *node,
> >  	return 0;
> >  }
> >  
> > +static inline bool of_pci_bridge_d3(struct device_node *node)
> > +{
> > +	return false;
> > +}
> > +
> >  static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
> >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > 
> > ---
> > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > change-id: 20240131-pcie-qcom-bridge-b6802a9770a3
> > 
> > Best regards,
> > -- 
> > Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> >
  

On Wed, Feb 21, 2024 at 10:49:58AM +0530, Manivannan Sadhasivam wrote:
> On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> > On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > > Currently, PCI core will enable D3 support for PCI bridges only when the
> > > following conditions are met:
> > 
> > Whenever I read "D3", I first have to figure out whether we're talking
> > about D3hot or D3cold.  Please save me the effort :)
> 
> Both actually, that's why I used "D3" as in the spec. I should've explicitly
> mentioned that in the commit message.
> 
> > > 1. Platform is ACPI based
> > > 2. Thunderbolt controller is used
> > > 3. pcie_port_pm=force passed in cmdline
> > 
> > Are these joined by "AND" or "OR"?  I guess probably "OR"?
> > 
> > "... all the following conditions are met" or "... one of the
> > following conditions is met" would clarify this.
> 
> Will use "one of the..."
> 
> > > While options 1 and 2 do not apply to most of the DT based
> > > platforms, option 3 will make the life harder for distro
> > > maintainers. Due to this, runtime PM is also not getting enabled
> > > for the bridges.
> > > 
> > > To fix this, let's make use of the "supports-d3" property [1] in
> > > the bridge DT nodes to enable D3 support for the capable
> > > bridges. This will also allow the capable bridges to support
> > > runtime PM, thereby conserving power.
> > 
> > Looks like "supports-d3" was added by
> > https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> > The commit log mentions "platform specific ways", which suggests maybe
> > this is D3cold, since D3hot should be supported via PMCSR without any
> > help from the platform.
> > 
> > So I *guess* this really means "platform provides some non-architected
> > way to put devices in D3cold and bring them back to D0"?
> 
> By reading the comments and git log of the pci_bridge_d3_possible()
> function in drivers/pci/pci.c, we can understand that some of the
> old bridges do not support both D3hot and D3cold. And to
> differentiate such bridges, platforms have to notify the OS using
> some ways.
> 
> ACPI has its own implementation [1] and DT uses "supports-d3"
> property.
> 
> And yes, in an ideal world PMCSR should be sufficient for D3hot, but
> you know the PCI vendors more than me ;)

So it sounds like this is supposed to cover two cases:

  1) D3hot doesn't work per spec.  This sounds like a hardware
     defect in the device that should be a quirk based on
     Vendor/Device ID, not something in DT.  I don't actually know if
     this is common, although there are several existing quirks that
     mention issues with D3.

  2) The platform doesn't support putting the bridge in D3cold and
     back to D0.  I don't understand this either because I assumed DT
     would describe *hardware*, and "supports-d3" might imply the
     presence of hardware power control, but doesn't tell us how to
     operate it, and it must be up to a native driver to know how to
     do it.

These are two vastly different scenarios, and I would really like to
untangle them so they aren't conflated.  I see that you're extending
platform_pci_bridge_d3(), which apparently has that conflation baked
into it already, but my personal experience is that this is really
hard to maintain.

> > > Ideally, D3 support should be enabled by default for the more recent PCI
> > > bridges, but we do not have a sane way to detect them.
> > > 
> > > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31

> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci-acpi.c#n976
> 
> > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > ---
> > > This patch is tested on Qcom SM8450 based development board with an out-of-tree
> > > DT patch.
> > > 
> > > NOTE: I will submit the DT patches adding this property for applicable bridges
> > > in Qcom SoCs separately.
> > > 
> > > Changes in v3:
> > > - Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
> > > - Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org
> > > 
> > > Changes in v2:
> > > - Switched to DT based approach as suggested by Lukas.
> > > - Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
> > > ---
> > >  drivers/pci/of.c  | 12 ++++++++++++
> > >  drivers/pci/pci.c |  3 +++
> > >  drivers/pci/pci.h |  6 ++++++
> > >  3 files changed, 21 insertions(+)
> > > 
> > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > index 51e3dd0ea5ab..24b0107802af 100644
> > > --- a/drivers/pci/of.c
> > > +++ b/drivers/pci/of.c
> > > @@ -786,3 +786,15 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> > >  	return slot_power_limit_mw;
> > >  }
> > >  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > > +
> > > +/**
> > > + * of_pci_bridge_d3 - Check if the bridge is supporting D3 states or not
> > > + *
> > > + * @node: device tree node of the bridge
> > > + *
> > > + * Return: %true if the bridge is supporting D3 states, %false otherwise.
> > > + */
> > > +bool of_pci_bridge_d3(struct device_node *node)
> > > +{
> > > +	return of_property_present(node, "supports-d3");
> > > +}
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index d8f11a078924..8678fba092bb 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -1142,6 +1142,9 @@ static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> > >  	if (pci_use_mid_pm())
> > >  		return false;
> > >  
> > > +	if (dev_of_node(&dev->dev))
> > > +		return of_pci_bridge_d3(dev->dev.of_node);
> > > +
> > >  	return acpi_pci_bridge_d3(dev);
> > >  }
> > >  
> > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > index 2336a8d1edab..10387461b1fe 100644
> > > --- a/drivers/pci/pci.h
> > > +++ b/drivers/pci/pci.h
> > > @@ -635,6 +635,7 @@ int of_pci_get_max_link_speed(struct device_node *node);
> > >  u32 of_pci_get_slot_power_limit(struct device_node *node,
> > >  				u8 *slot_power_limit_value,
> > >  				u8 *slot_power_limit_scale);
> > > +bool of_pci_bridge_d3(struct device_node *node);
> > >  int pci_set_of_node(struct pci_dev *dev);
> > >  void pci_release_of_node(struct pci_dev *dev);
> > >  void pci_set_bus_of_node(struct pci_bus *bus);
> > > @@ -673,6 +674,11 @@ of_pci_get_slot_power_limit(struct device_node *node,
> > >  	return 0;
> > >  }
> > >  
> > > +static inline bool of_pci_bridge_d3(struct device_node *node)
> > > +{
> > > +	return false;
> > > +}
> > > +
> > >  static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
> > >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> > >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > > 
> > > ---
> > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > > change-id: 20240131-pcie-qcom-bridge-b6802a9770a3
> > > 
> > > Best regards,
> > > -- 
> > > Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > 
> 
> -- 
> மணிவண்ணன் சதாசிவம்
  

On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> On Wed, Feb 21, 2024 at 10:49:58AM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> > > On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > > > Currently, PCI core will enable D3 support for PCI bridges only when the
> > > > following conditions are met:
> > > 
> > > Whenever I read "D3", I first have to figure out whether we're talking
> > > about D3hot or D3cold.  Please save me the effort :)
> > 
> > Both actually, that's why I used "D3" as in the spec. I should've explicitly
> > mentioned that in the commit message.
> > 
> > > > 1. Platform is ACPI based
> > > > 2. Thunderbolt controller is used
> > > > 3. pcie_port_pm=force passed in cmdline
> > > 
> > > Are these joined by "AND" or "OR"?  I guess probably "OR"?
> > > 
> > > "... all the following conditions are met" or "... one of the
> > > following conditions is met" would clarify this.
> > 
> > Will use "one of the..."
> > 
> > > > While options 1 and 2 do not apply to most of the DT based
> > > > platforms, option 3 will make the life harder for distro
> > > > maintainers. Due to this, runtime PM is also not getting enabled
> > > > for the bridges.
> > > > 
> > > > To fix this, let's make use of the "supports-d3" property [1] in
> > > > the bridge DT nodes to enable D3 support for the capable
> > > > bridges. This will also allow the capable bridges to support
> > > > runtime PM, thereby conserving power.
> > > 
> > > Looks like "supports-d3" was added by
> > > https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> > > The commit log mentions "platform specific ways", which suggests maybe
> > > this is D3cold, since D3hot should be supported via PMCSR without any
> > > help from the platform.
> > > 
> > > So I *guess* this really means "platform provides some non-architected
> > > way to put devices in D3cold and bring them back to D0"?
> > 
> > By reading the comments and git log of the pci_bridge_d3_possible()
> > function in drivers/pci/pci.c, we can understand that some of the
> > old bridges do not support both D3hot and D3cold. And to
> > differentiate such bridges, platforms have to notify the OS using
> > some ways.
> > 
> > ACPI has its own implementation [1] and DT uses "supports-d3"
> > property.
> > 
> > And yes, in an ideal world PMCSR should be sufficient for D3hot, but
> > you know the PCI vendors more than me ;)
> 
> So it sounds like this is supposed to cover two cases:
> 
>   1) D3hot doesn't work per spec.  This sounds like a hardware
>      defect in the device that should be a quirk based on
>      Vendor/Device ID, not something in DT.  I don't actually know if
>      this is common, although there are several existing quirks that
>      mention issues with D3.
> 

I'd love to use quirks if we started from that. But right now, quirks are not
used and there are multiple checks based on various factors [1], including
relying on ACPI. So that's the reason I went with DT based approach.

If quirks has to be used now, then it has to be used for both ACPI and DT based
platforms. For DT it won't be an issue since nobody bothered until now, but for
ACPI, we need to add quirks for all the bridges in the wild which is not
feasible.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci.c#n3116

>   2) The platform doesn't support putting the bridge in D3cold and
>      back to D0.  I don't understand this either because I assumed DT
>      would describe *hardware*, and "supports-d3" might imply the
>      presence of hardware power control, but doesn't tell us how to
>      operate it, and it must be up to a native driver to know how to
>      do it.
> 

"supports-d3" implies that both D3hot and D3cold works as in the spec and the OS
can handle it appropriately. If this is absent, then OS should not transition
the bridge to any of the D3 states. I don't understand what is the confusion
here. This is similar to what we already have for ACPI (whether or not it is
correct is another topic).

> These are two vastly different scenarios, and I would really like to
> untangle them so they aren't conflated.  I see that you're extending
> platform_pci_bridge_d3(), which apparently has that conflation baked
> into it already, but my personal experience is that this is really
> hard to maintain.
> 

I do agree that it is not in a good shape, but there is no better solution other
than making use of the DT property. If you have any better idea, please suggest.

- Mani

> > > > Ideally, D3 support should be enabled by default for the more recent PCI
> > > > bridges, but we do not have a sane way to detect them.
> > > > 
> > > > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31
> 
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci-acpi.c#n976
> > 
> > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > ---
> > > > This patch is tested on Qcom SM8450 based development board with an out-of-tree
> > > > DT patch.
> > > > 
> > > > NOTE: I will submit the DT patches adding this property for applicable bridges
> > > > in Qcom SoCs separately.
> > > > 
> > > > Changes in v3:
> > > > - Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
> > > > - Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org
> > > > 
> > > > Changes in v2:
> > > > - Switched to DT based approach as suggested by Lukas.
> > > > - Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
> > > > ---
> > > >  drivers/pci/of.c  | 12 ++++++++++++
> > > >  drivers/pci/pci.c |  3 +++
> > > >  drivers/pci/pci.h |  6 ++++++
> > > >  3 files changed, 21 insertions(+)
> > > > 
> > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > > index 51e3dd0ea5ab..24b0107802af 100644
> > > > --- a/drivers/pci/of.c
> > > > +++ b/drivers/pci/of.c
> > > > @@ -786,3 +786,15 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > >  	return slot_power_limit_mw;
> > > >  }
> > > >  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > > > +
> > > > +/**
> > > > + * of_pci_bridge_d3 - Check if the bridge is supporting D3 states or not
> > > > + *
> > > > + * @node: device tree node of the bridge
> > > > + *
> > > > + * Return: %true if the bridge is supporting D3 states, %false otherwise.
> > > > + */
> > > > +bool of_pci_bridge_d3(struct device_node *node)
> > > > +{
> > > > +	return of_property_present(node, "supports-d3");
> > > > +}
> > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > index d8f11a078924..8678fba092bb 100644
> > > > --- a/drivers/pci/pci.c
> > > > +++ b/drivers/pci/pci.c
> > > > @@ -1142,6 +1142,9 @@ static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> > > >  	if (pci_use_mid_pm())
> > > >  		return false;
> > > >  
> > > > +	if (dev_of_node(&dev->dev))
> > > > +		return of_pci_bridge_d3(dev->dev.of_node);
> > > > +
> > > >  	return acpi_pci_bridge_d3(dev);
> > > >  }
> > > >  
> > > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > > index 2336a8d1edab..10387461b1fe 100644
> > > > --- a/drivers/pci/pci.h
> > > > +++ b/drivers/pci/pci.h
> > > > @@ -635,6 +635,7 @@ int of_pci_get_max_link_speed(struct device_node *node);
> > > >  u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > >  				u8 *slot_power_limit_value,
> > > >  				u8 *slot_power_limit_scale);
> > > > +bool of_pci_bridge_d3(struct device_node *node);
> > > >  int pci_set_of_node(struct pci_dev *dev);
> > > >  void pci_release_of_node(struct pci_dev *dev);
> > > >  void pci_set_bus_of_node(struct pci_bus *bus);
> > > > @@ -673,6 +674,11 @@ of_pci_get_slot_power_limit(struct device_node *node,
> > > >  	return 0;
> > > >  }
> > > >  
> > > > +static inline bool of_pci_bridge_d3(struct device_node *node)
> > > > +{
> > > > +	return false;
> > > > +}
> > > > +
> > > >  static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
> > > >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> > > >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > > > 
> > > > ---
> > > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > > > change-id: 20240131-pcie-qcom-bridge-b6802a9770a3
> > > > 
> > > > Best regards,
> > > > -- 
> > > > Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > 
> > 
> > -- 
> > மணிவண்ணன் சதாசிவம்
  

On Thu, Feb 22, 2024 at 09:36:29AM +0530, Manivannan Sadhasivam wrote:
> On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> > On Wed, Feb 21, 2024 at 10:49:58AM +0530, Manivannan Sadhasivam wrote:
> > > On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> > > > On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > > > > Currently, PCI core will enable D3 support for PCI bridges only when the
> > > > > following conditions are met:
> > > > 
> > > > Whenever I read "D3", I first have to figure out whether we're talking
> > > > about D3hot or D3cold.  Please save me the effort :)
> > > 
> > > Both actually, that's why I used "D3" as in the spec. I should've explicitly
> > > mentioned that in the commit message.
> > > 
> > > > > 1. Platform is ACPI based
> > > > > 2. Thunderbolt controller is used
> > > > > 3. pcie_port_pm=force passed in cmdline
> > > > 
> > > > Are these joined by "AND" or "OR"?  I guess probably "OR"?
> > > > 
> > > > "... all the following conditions are met" or "... one of the
> > > > following conditions is met" would clarify this.
> > > 
> > > Will use "one of the..."
> > > 
> > > > > While options 1 and 2 do not apply to most of the DT based
> > > > > platforms, option 3 will make the life harder for distro
> > > > > maintainers. Due to this, runtime PM is also not getting enabled
> > > > > for the bridges.
> > > > > 
> > > > > To fix this, let's make use of the "supports-d3" property [1] in
> > > > > the bridge DT nodes to enable D3 support for the capable
> > > > > bridges. This will also allow the capable bridges to support
> > > > > runtime PM, thereby conserving power.
> > > > 
> > > > Looks like "supports-d3" was added by
> > > > https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> > > > The commit log mentions "platform specific ways", which suggests maybe
> > > > this is D3cold, since D3hot should be supported via PMCSR without any
> > > > help from the platform.
> > > > 
> > > > So I *guess* this really means "platform provides some non-architected
> > > > way to put devices in D3cold and bring them back to D0"?
> > > 
> > > By reading the comments and git log of the pci_bridge_d3_possible()
> > > function in drivers/pci/pci.c, we can understand that some of the
> > > old bridges do not support both D3hot and D3cold. And to
> > > differentiate such bridges, platforms have to notify the OS using
> > > some ways.
> > > 
> > > ACPI has its own implementation [1] and DT uses "supports-d3"
> > > property.
> > > 
> > > And yes, in an ideal world PMCSR should be sufficient for D3hot, but
> > > you know the PCI vendors more than me ;)
> > 
> > So it sounds like this is supposed to cover two cases:
> > 
> >   1) D3hot doesn't work per spec.  This sounds like a hardware
> >      defect in the device that should be a quirk based on
> >      Vendor/Device ID, not something in DT.  I don't actually know if
> >      this is common, although there are several existing quirks that
> >      mention issues with D3.
> > 
> 
> I'd love to use quirks if we started from that. But right now, quirks are not
> used and there are multiple checks based on various factors [1], including
> relying on ACPI. So that's the reason I went with DT based approach.
> 
> If quirks has to be used now, then it has to be used for both ACPI and DT based
> platforms. For DT it won't be an issue since nobody bothered until now, but for
> ACPI, we need to add quirks for all the bridges in the wild which is not
> feasible.
> 
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci.c#n3116
> 
> >   2) The platform doesn't support putting the bridge in D3cold and
> >      back to D0.  I don't understand this either because I assumed DT
> >      would describe *hardware*, and "supports-d3" might imply the
> >      presence of hardware power control, but doesn't tell us how to
> >      operate it, and it must be up to a native driver to know how to
> >      do it.
> 
> "supports-d3" implies that both D3hot and D3cold works as in the
> spec and the OS can handle it appropriately. If this is absent, then
> OS should not transition the bridge to any of the D3 states. I don't
> understand what is the confusion here. This is similar to what we
> already have for ACPI (whether or not it is correct is another
> topic).

What does "the OS can handle it appropriately" mean?  Whatever it
means, it sounds like a property of the OS, not a property of the
device.

I don't know what "D3cold works as in the spec" means, either.  The
spec says how D3cold affects internal device state, but it doesn't say
anything about how to put devices in D3cold or back in D0.

> > These are two vastly different scenarios, and I would really like to
> > untangle them so they aren't conflated.  I see that you're extending
> > platform_pci_bridge_d3(), which apparently has that conflation baked
> > into it already, but my personal experience is that this is really
> > hard to maintain.
> 
> I do agree that it is not in a good shape, but there is no better
> solution other than making use of the DT property. If you have any
> better idea, please suggest.

The longer this goes on the worse shape we are in because we're always
adding new special cases.

The fundamental problem I have is that "supports-d3" doesn't say
anything specific other than "current Linux can put the device in
D3hot or D3cold and get it back out again".  I think DT should tell us
characteristics of the device or the platform, e.g., "PMCSR doesn't
work to enter/leave D3hot on this device" or "regulator X controls
main power to the device to enter/leave D3cold".

But right now it sounds like a mixture of "PMCSR works correctly to
enter/leave D3hot" and "some unspecified software can control main
power to this device".

Putting devices in D3cold and back in D0 needs some kind of platform
support like ACPI methods or a native power management driver that
knows how to control power on a specific platform.  That's completely
different from D3hot, where the PCI spec tells us all we need to know.

> > > > > Ideally, D3 support should be enabled by default for the more recent PCI
> > > > > bridges, but we do not have a sane way to detect them.
> > > > > 
> > > > > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31
> > 
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci-acpi.c#n976
> > > 
> > > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > > ---
> > > > > This patch is tested on Qcom SM8450 based development board with an out-of-tree
> > > > > DT patch.
> > > > > 
> > > > > NOTE: I will submit the DT patches adding this property for applicable bridges
> > > > > in Qcom SoCs separately.
> > > > > 
> > > > > Changes in v3:
> > > > > - Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
> > > > > - Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org
> > > > > 
> > > > > Changes in v2:
> > > > > - Switched to DT based approach as suggested by Lukas.
> > > > > - Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
> > > > > ---
> > > > >  drivers/pci/of.c  | 12 ++++++++++++
> > > > >  drivers/pci/pci.c |  3 +++
> > > > >  drivers/pci/pci.h |  6 ++++++
> > > > >  3 files changed, 21 insertions(+)
> > > > > 
> > > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > > > index 51e3dd0ea5ab..24b0107802af 100644
> > > > > --- a/drivers/pci/of.c
> > > > > +++ b/drivers/pci/of.c
> > > > > @@ -786,3 +786,15 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > > >  	return slot_power_limit_mw;
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > > > > +
> > > > > +/**
> > > > > + * of_pci_bridge_d3 - Check if the bridge is supporting D3 states or not
> > > > > + *
> > > > > + * @node: device tree node of the bridge
> > > > > + *
> > > > > + * Return: %true if the bridge is supporting D3 states, %false otherwise.
> > > > > + */
> > > > > +bool of_pci_bridge_d3(struct device_node *node)
> > > > > +{
> > > > > +	return of_property_present(node, "supports-d3");
> > > > > +}
> > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > > index d8f11a078924..8678fba092bb 100644
> > > > > --- a/drivers/pci/pci.c
> > > > > +++ b/drivers/pci/pci.c
> > > > > @@ -1142,6 +1142,9 @@ static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> > > > >  	if (pci_use_mid_pm())
> > > > >  		return false;
> > > > >  
> > > > > +	if (dev_of_node(&dev->dev))
> > > > > +		return of_pci_bridge_d3(dev->dev.of_node);
> > > > > +
> > > > >  	return acpi_pci_bridge_d3(dev);
> > > > >  }
> > > > >  
> > > > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > > > index 2336a8d1edab..10387461b1fe 100644
> > > > > --- a/drivers/pci/pci.h
> > > > > +++ b/drivers/pci/pci.h
> > > > > @@ -635,6 +635,7 @@ int of_pci_get_max_link_speed(struct device_node *node);
> > > > >  u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > > >  				u8 *slot_power_limit_value,
> > > > >  				u8 *slot_power_limit_scale);
> > > > > +bool of_pci_bridge_d3(struct device_node *node);
> > > > >  int pci_set_of_node(struct pci_dev *dev);
> > > > >  void pci_release_of_node(struct pci_dev *dev);
> > > > >  void pci_set_bus_of_node(struct pci_bus *bus);
> > > > > @@ -673,6 +674,11 @@ of_pci_get_slot_power_limit(struct device_node *node,
> > > > >  	return 0;
> > > > >  }
> > > > >  
> > > > > +static inline bool of_pci_bridge_d3(struct device_node *node)
> > > > > +{
> > > > > +	return false;
> > > > > +}
> > > > > +
> > > > >  static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
> > > > >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> > > > >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > > > > 
> > > > > ---
> > > > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > > > > change-id: 20240131-pcie-qcom-bridge-b6802a9770a3
> > > > > 
> > > > > Best regards,
> > > > > -- 
> > > > > Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > > 
> > > 
> > > -- 
> > > மணிவண்ணன் சதாசிவம்
> 
> -- 
> மணிவண்ணன் சதாசிவம்
  

On Mon, Feb 26, 2024 at 05:39:30PM -0600, Bjorn Helgaas wrote:
> On Thu, Feb 22, 2024 at 09:36:29AM +0530, Manivannan Sadhasivam wrote:
> > On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> > > On Wed, Feb 21, 2024 at 10:49:58AM +0530, Manivannan Sadhasivam wrote:
> > > > On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> > > > > On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > > > > > Currently, PCI core will enable D3 support for PCI bridges only when the
> > > > > > following conditions are met:
> > > > > 
> > > > > Whenever I read "D3", I first have to figure out whether we're talking
> > > > > about D3hot or D3cold.  Please save me the effort :)
> > > > 
> > > > Both actually, that's why I used "D3" as in the spec. I should've explicitly
> > > > mentioned that in the commit message.
> > > > 
> > > > > > 1. Platform is ACPI based
> > > > > > 2. Thunderbolt controller is used
> > > > > > 3. pcie_port_pm=force passed in cmdline
> > > > > 
> > > > > Are these joined by "AND" or "OR"?  I guess probably "OR"?
> > > > > 
> > > > > "... all the following conditions are met" or "... one of the
> > > > > following conditions is met" would clarify this.
> > > > 
> > > > Will use "one of the..."
> > > > 
> > > > > > While options 1 and 2 do not apply to most of the DT based
> > > > > > platforms, option 3 will make the life harder for distro
> > > > > > maintainers. Due to this, runtime PM is also not getting enabled
> > > > > > for the bridges.
> > > > > > 
> > > > > > To fix this, let's make use of the "supports-d3" property [1] in
> > > > > > the bridge DT nodes to enable D3 support for the capable
> > > > > > bridges. This will also allow the capable bridges to support
> > > > > > runtime PM, thereby conserving power.
> > > > > 
> > > > > Looks like "supports-d3" was added by
> > > > > https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> > > > > The commit log mentions "platform specific ways", which suggests maybe
> > > > > this is D3cold, since D3hot should be supported via PMCSR without any
> > > > > help from the platform.
> > > > > 
> > > > > So I *guess* this really means "platform provides some non-architected
> > > > > way to put devices in D3cold and bring them back to D0"?
> > > > 
> > > > By reading the comments and git log of the pci_bridge_d3_possible()
> > > > function in drivers/pci/pci.c, we can understand that some of the
> > > > old bridges do not support both D3hot and D3cold. And to
> > > > differentiate such bridges, platforms have to notify the OS using
> > > > some ways.
> > > > 
> > > > ACPI has its own implementation [1] and DT uses "supports-d3"
> > > > property.
> > > > 
> > > > And yes, in an ideal world PMCSR should be sufficient for D3hot, but
> > > > you know the PCI vendors more than me ;)
> > > 
> > > So it sounds like this is supposed to cover two cases:
> > > 
> > >   1) D3hot doesn't work per spec.  This sounds like a hardware
> > >      defect in the device that should be a quirk based on
> > >      Vendor/Device ID, not something in DT.  I don't actually know if
> > >      this is common, although there are several existing quirks that
> > >      mention issues with D3.
> > > 
> > 
> > I'd love to use quirks if we started from that. But right now, quirks are not
> > used and there are multiple checks based on various factors [1], including
> > relying on ACPI. So that's the reason I went with DT based approach.
> > 
> > If quirks has to be used now, then it has to be used for both ACPI and DT based
> > platforms. For DT it won't be an issue since nobody bothered until now, but for
> > ACPI, we need to add quirks for all the bridges in the wild which is not
> > feasible.
> > 
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci.c#n3116
> > 
> > >   2) The platform doesn't support putting the bridge in D3cold and
> > >      back to D0.  I don't understand this either because I assumed DT
> > >      would describe *hardware*, and "supports-d3" might imply the
> > >      presence of hardware power control, but doesn't tell us how to
> > >      operate it, and it must be up to a native driver to know how to
> > >      do it.
> > 
> > "supports-d3" implies that both D3hot and D3cold works as in the
> > spec and the OS can handle it appropriately. If this is absent, then
> > OS should not transition the bridge to any of the D3 states. I don't
> > understand what is the confusion here. This is similar to what we
> > already have for ACPI (whether or not it is correct is another
> > topic).
> 
> What does "the OS can handle it appropriately" mean?  Whatever it
> means, it sounds like a property of the OS, not a property of the
> device.
> 

"appropiately" means as per the PCIe spec.

> I don't know what "D3cold works as in the spec" means, either.  The
> spec says how D3cold affects internal device state, but it doesn't say
> anything about how to put devices in D3cold or back in D0.
> 
> > > These are two vastly different scenarios, and I would really like to
> > > untangle them so they aren't conflated.  I see that you're extending
> > > platform_pci_bridge_d3(), which apparently has that conflation baked
> > > into it already, but my personal experience is that this is really
> > > hard to maintain.
> > 
> > I do agree that it is not in a good shape, but there is no better
> > solution other than making use of the DT property. If you have any
> > better idea, please suggest.
> 
> The longer this goes on the worse shape we are in because we're always
> adding new special cases.
> 
> The fundamental problem I have is that "supports-d3" doesn't say
> anything specific other than "current Linux can put the device in
> D3hot or D3cold and get it back out again".  I think DT should tell us
> characteristics of the device or the platform, e.g., "PMCSR doesn't
> work to enter/leave D3hot on this device" or "regulator X controls
> main power to the device to enter/leave D3cold".
> 
> But right now it sounds like a mixture of "PMCSR works correctly to
> enter/leave D3hot" and "some unspecified software can control main
> power to this device".
> 
> Putting devices in D3cold and back in D0 needs some kind of platform
> support like ACPI methods or a native power management driver that
> knows how to control power on a specific platform.  That's completely
> different from D3hot, where the PCI spec tells us all we need to know.
> 

Ok, I got the issue. TBH, I added the device tree property based on the existing
quirks for the ACPI devices. But none of the DT based platforms I'm aware of
(even the legacy Qcom MSM8996 chipset released in early 2016) doesn't have any
issue with D3hot. But I'm just nervous to assume it is the case for all the DT
based platforms in the wild.

But to proceed further, what is your preference? Should we ammend the DT
property to make it explicit that the propery only focuses on the D3hot
capability of the bridge and it works as per the spec (PMCSR) or bite the bullet
and enable D3hot for all the non-ACPI platforms?

We can add quirks for the bridges later on if we happen to receive any bug
report.

- Mani

> > > > > > Ideally, D3 support should be enabled by default for the more recent PCI
> > > > > > bridges, but we do not have a sane way to detect them.
> > > > > > 
> > > > > > [1] https://github.com/devicetree-org/dt-schema/blob/main/dtschema/schemas/pci/pci-pci-bridge.yaml#L31
> > > 
> > > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci-acpi.c#n976
> > > > 
> > > > > > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > > > ---
> > > > > > This patch is tested on Qcom SM8450 based development board with an out-of-tree
> > > > > > DT patch.
> > > > > > 
> > > > > > NOTE: I will submit the DT patches adding this property for applicable bridges
> > > > > > in Qcom SoCs separately.
> > > > > > 
> > > > > > Changes in v3:
> > > > > > - Fixed kdoc, used of_property_present() and dev_of_node() (Lukas)
> > > > > > - Link to v2: https://lore.kernel.org/r/20240214-pcie-qcom-bridge-v2-1-9dd6dbb1b817@linaro.org
> > > > > > 
> > > > > > Changes in v2:
> > > > > > - Switched to DT based approach as suggested by Lukas.
> > > > > > - Link to v1: https://lore.kernel.org/r/20240202-pcie-qcom-bridge-v1-0-46d7789836c0@linaro.org
> > > > > > ---
> > > > > >  drivers/pci/of.c  | 12 ++++++++++++
> > > > > >  drivers/pci/pci.c |  3 +++
> > > > > >  drivers/pci/pci.h |  6 ++++++
> > > > > >  3 files changed, 21 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/pci/of.c b/drivers/pci/of.c
> > > > > > index 51e3dd0ea5ab..24b0107802af 100644
> > > > > > --- a/drivers/pci/of.c
> > > > > > +++ b/drivers/pci/of.c
> > > > > > @@ -786,3 +786,15 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > > > >  	return slot_power_limit_mw;
> > > > > >  }
> > > > > >  EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
> > > > > > +
> > > > > > +/**
> > > > > > + * of_pci_bridge_d3 - Check if the bridge is supporting D3 states or not
> > > > > > + *
> > > > > > + * @node: device tree node of the bridge
> > > > > > + *
> > > > > > + * Return: %true if the bridge is supporting D3 states, %false otherwise.
> > > > > > + */
> > > > > > +bool of_pci_bridge_d3(struct device_node *node)
> > > > > > +{
> > > > > > +	return of_property_present(node, "supports-d3");
> > > > > > +}
> > > > > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > > > > index d8f11a078924..8678fba092bb 100644
> > > > > > --- a/drivers/pci/pci.c
> > > > > > +++ b/drivers/pci/pci.c
> > > > > > @@ -1142,6 +1142,9 @@ static inline bool platform_pci_bridge_d3(struct pci_dev *dev)
> > > > > >  	if (pci_use_mid_pm())
> > > > > >  		return false;
> > > > > >  
> > > > > > +	if (dev_of_node(&dev->dev))
> > > > > > +		return of_pci_bridge_d3(dev->dev.of_node);
> > > > > > +
> > > > > >  	return acpi_pci_bridge_d3(dev);
> > > > > >  }
> > > > > >  
> > > > > > diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> > > > > > index 2336a8d1edab..10387461b1fe 100644
> > > > > > --- a/drivers/pci/pci.h
> > > > > > +++ b/drivers/pci/pci.h
> > > > > > @@ -635,6 +635,7 @@ int of_pci_get_max_link_speed(struct device_node *node);
> > > > > >  u32 of_pci_get_slot_power_limit(struct device_node *node,
> > > > > >  				u8 *slot_power_limit_value,
> > > > > >  				u8 *slot_power_limit_scale);
> > > > > > +bool of_pci_bridge_d3(struct device_node *node);
> > > > > >  int pci_set_of_node(struct pci_dev *dev);
> > > > > >  void pci_release_of_node(struct pci_dev *dev);
> > > > > >  void pci_set_bus_of_node(struct pci_bus *bus);
> > > > > > @@ -673,6 +674,11 @@ of_pci_get_slot_power_limit(struct device_node *node,
> > > > > >  	return 0;
> > > > > >  }
> > > > > >  
> > > > > > +static inline bool of_pci_bridge_d3(struct device_node *node)
> > > > > > +{
> > > > > > +	return false;
> > > > > > +}
> > > > > > +
> > > > > >  static inline int pci_set_of_node(struct pci_dev *dev) { return 0; }
> > > > > >  static inline void pci_release_of_node(struct pci_dev *dev) { }
> > > > > >  static inline void pci_set_bus_of_node(struct pci_bus *bus) { }
> > > > > > 
> > > > > > ---
> > > > > > base-commit: 6613476e225e090cc9aad49be7fa504e290dd33d
> > > > > > change-id: 20240131-pcie-qcom-bridge-b6802a9770a3
> > > > > > 
> > > > > > Best regards,
> > > > > > -- 
> > > > > > Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
> > > > > > 
> > > > 
> > > > -- 
> > > > மணிவண்ணன் சதாசிவம்
> > 
> > -- 
> > மணிவண்ணன் சதாசிவம்
  

On Tue, Feb 27, 2024 at 01:00:57PM +0530, Manivannan Sadhasivam wrote:
> On Mon, Feb 26, 2024 at 05:39:30PM -0600, Bjorn Helgaas wrote:
> > On Thu, Feb 22, 2024 at 09:36:29AM +0530, Manivannan Sadhasivam wrote:
> > > On Wed, Feb 21, 2024 at 12:20:00PM -0600, Bjorn Helgaas wrote:
> > > > On Wed, Feb 21, 2024 at 10:49:58AM +0530, Manivannan Sadhasivam wrote:
> > > > > On Tue, Feb 20, 2024 at 04:02:40PM -0600, Bjorn Helgaas wrote:
> > > > > > On Wed, Feb 14, 2024 at 05:16:09PM +0530, Manivannan Sadhasivam wrote:
> > > > > > > Currently, PCI core will enable D3 support for PCI bridges only when the
> > > > > > > following conditions are met:
> > > > > > 
> > > > > > Whenever I read "D3", I first have to figure out whether we're talking
> > > > > > about D3hot or D3cold.  Please save me the effort :)
> > > > > 
> > > > > Both actually, that's why I used "D3" as in the spec. I should've explicitly
> > > > > mentioned that in the commit message.
> > > > > 
> > > > > > > 1. Platform is ACPI based
> > > > > > > 2. Thunderbolt controller is used
> > > > > > > 3. pcie_port_pm=force passed in cmdline
> > > > > > 
> > > > > > Are these joined by "AND" or "OR"?  I guess probably "OR"?
> > > > > > 
> > > > > > "... all the following conditions are met" or "... one of the
> > > > > > following conditions is met" would clarify this.
> > > > > 
> > > > > Will use "one of the..."
> > > > > 
> > > > > > > While options 1 and 2 do not apply to most of the DT based
> > > > > > > platforms, option 3 will make the life harder for distro
> > > > > > > maintainers. Due to this, runtime PM is also not getting enabled
> > > > > > > for the bridges.
> > > > > > > 
> > > > > > > To fix this, let's make use of the "supports-d3" property [1] in
> > > > > > > the bridge DT nodes to enable D3 support for the capable
> > > > > > > bridges. This will also allow the capable bridges to support
> > > > > > > runtime PM, thereby conserving power.
> > > > > > 
> > > > > > Looks like "supports-d3" was added by
> > > > > > https://github.com/devicetree-org/dt-schema/commit/4548397d7522.
> > > > > > The commit log mentions "platform specific ways", which suggests maybe
> > > > > > this is D3cold, since D3hot should be supported via PMCSR without any
> > > > > > help from the platform.
> > > > > > 
> > > > > > So I *guess* this really means "platform provides some non-architected
> > > > > > way to put devices in D3cold and bring them back to D0"?
> > > > > 
> > > > > By reading the comments and git log of the pci_bridge_d3_possible()
> > > > > function in drivers/pci/pci.c, we can understand that some of the
> > > > > old bridges do not support both D3hot and D3cold. And to
> > > > > differentiate such bridges, platforms have to notify the OS using
> > > > > some ways.
> > > > > 
> > > > > ACPI has its own implementation [1] and DT uses "supports-d3"
> > > > > property.
> > > > > 
> > > > > And yes, in an ideal world PMCSR should be sufficient for D3hot, but
> > > > > you know the PCI vendors more than me ;)
> > > > 
> > > > So it sounds like this is supposed to cover two cases:
> > > > 
> > > >   1) D3hot doesn't work per spec.  This sounds like a hardware
> > > >      defect in the device that should be a quirk based on
> > > >      Vendor/Device ID, not something in DT.  I don't actually know if
> > > >      this is common, although there are several existing quirks that
> > > >      mention issues with D3.
> > > > 
> > > 
> > > I'd love to use quirks if we started from that. But right now, quirks are not
> > > used and there are multiple checks based on various factors [1], including
> > > relying on ACPI. So that's the reason I went with DT based approach.
> > > 
> > > If quirks has to be used now, then it has to be used for both ACPI and DT based
> > > platforms. For DT it won't be an issue since nobody bothered until now, but for
> > > ACPI, we need to add quirks for all the bridges in the wild which is not
> > > feasible.
> > > 
> > > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/pci/pci.c#n3116
> > > 
> > > >   2) The platform doesn't support putting the bridge in D3cold and
> > > >      back to D0.  I don't understand this either because I assumed DT
> > > >      would describe *hardware*, and "supports-d3" might imply the
> > > >      presence of hardware power control, but doesn't tell us how to
> > > >      operate it, and it must be up to a native driver to know how to
> > > >      do it.
> > > 
> > > "supports-d3" implies that both D3hot and D3cold works as in the
> > > spec and the OS can handle it appropriately. If this is absent, then
> > > OS should not transition the bridge to any of the D3 states. I don't
> > > understand what is the confusion here. This is similar to what we
> > > already have for ACPI (whether or not it is correct is another
> > > topic).
> > 
> > What does "the OS can handle it appropriately" mean?  Whatever it
> > means, it sounds like a property of the OS, not a property of the
> > device.
> 
> "appropiately" means as per the PCIe spec.
>
> > I don't know what "D3cold works as in the spec" means, either.  The
> > spec says how D3cold affects internal device state, but it doesn't say
> > anything about how to put devices in D3cold or back in D0.
> > 
> > > > These are two vastly different scenarios, and I would really like to
> > > > untangle them so they aren't conflated.  I see that you're extending
> > > > platform_pci_bridge_d3(), which apparently has that conflation baked
> > > > into it already, but my personal experience is that this is really
> > > > hard to maintain.
> > > 
> > > I do agree that it is not in a good shape, but there is no better
> > > solution other than making use of the DT property. If you have any
> > > better idea, please suggest.
> > 
> > The longer this goes on the worse shape we are in because we're always
> > adding new special cases.
> > 
> > The fundamental problem I have is that "supports-d3" doesn't say
> > anything specific other than "current Linux can put the device in
> > D3hot or D3cold and get it back out again".  I think DT should tell us
> > characteristics of the device or the platform, e.g., "PMCSR doesn't
> > work to enter/leave D3hot on this device" or "regulator X controls
> > main power to the device to enter/leave D3cold".
> > 
> > But right now it sounds like a mixture of "PMCSR works correctly to
> > enter/leave D3hot" and "some unspecified software can control main
> > power to this device".
> > 
> > Putting devices in D3cold and back in D0 needs some kind of platform
> > support like ACPI methods or a native power management driver that
> > knows how to control power on a specific platform.  That's completely
> > different from D3hot, where the PCI spec tells us all we need to know.
> 
> Ok, I got the issue. TBH, I added the device tree property based on
> the existing quirks for the ACPI devices. But none of the DT based
> platforms I'm aware of (even the legacy Qcom MSM8996 chipset
> released in early 2016) doesn't have any issue with D3hot. But I'm
> just nervous to assume it is the case for all the DT based platforms
> in the wild.
> 
> But to proceed further, what is your preference? Should we ammend
> the DT property to make it explicit that the propery only focuses on
> the D3hot capability of the bridge and it works as per the spec
> (PMCSR) or bite the bullet and enable D3hot for all the non-ACPI
> platforms?
> 
> We can add quirks for the bridges later on if we happen to receive
> any bug report.

I would assume all devices support D3hot via PMCSR per spec.  We can
add quirks if we discover something that doesn't.

If we add annotations that "this device works correctly", we're
digging a hole for ourselves because it's impossible to remove those
annotations and they complicate all future maintenance.

Bjorn
  

On Tue, Feb 27, 2024 at 10:25:35AM -0600, Bjorn Helgaas wrote:

[...]

> > Ok, I got the issue. TBH, I added the device tree property based on
> > the existing quirks for the ACPI devices. But none of the DT based
> > platforms I'm aware of (even the legacy Qcom MSM8996 chipset
> > released in early 2016) doesn't have any issue with D3hot. But I'm
> > just nervous to assume it is the case for all the DT based platforms
> > in the wild.
> > 
> > But to proceed further, what is your preference? Should we ammend
> > the DT property to make it explicit that the propery only focuses on
> > the D3hot capability of the bridge and it works as per the spec
> > (PMCSR) or bite the bullet and enable D3hot for all the non-ACPI
> > platforms?
> > 
> > We can add quirks for the bridges later on if we happen to receive
> > any bug report.
> 
> I would assume all devices support D3hot via PMCSR per spec.  We can
> add quirks if we discover something that doesn't.
> 

When you say "all devices", are you referring to bridges in DT platforms or the
bridges across all platforms?

- Mani

> If we add annotations that "this device works correctly", we're
> digging a hole for ourselves because it's impossible to remove those
> annotations and they complicate all future maintenance.
> 
> Bjorn
  

On Tue, Feb 27, 2024 at 10:38:40PM +0530, Manivannan Sadhasivam wrote:
> On Tue, Feb 27, 2024 at 10:25:35AM -0600, Bjorn Helgaas wrote:
> 
> [...]
> 
> > > Ok, I got the issue. TBH, I added the device tree property based on
> > > the existing quirks for the ACPI devices. But none of the DT based
> > > platforms I'm aware of (even the legacy Qcom MSM8996 chipset
> > > released in early 2016) doesn't have any issue with D3hot. But I'm
> > > just nervous to assume it is the case for all the DT based platforms
> > > in the wild.
> > > 
> > > But to proceed further, what is your preference? Should we ammend
> > > the DT property to make it explicit that the propery only focuses on
> > > the D3hot capability of the bridge and it works as per the spec
> > > (PMCSR) or bite the bullet and enable D3hot for all the non-ACPI
> > > platforms?
> > > 
> > > We can add quirks for the bridges later on if we happen to receive
> > > any bug report.
> > 
> > I would assume all devices support D3hot via PMCSR per spec.  We can
> > add quirks if we discover something that doesn't.
> 
> When you say "all devices", are you referring to bridges in DT
> platforms or the bridges across all platforms?

This patch is only concerned with DT, so that's what I'm commenting on
here.  I don't know how to untangle the question of ACPI systems.

This patch affects platform_pci_bridge_d3(), so just based on the
"platform" in the function name, I would expect it to be concerned
with the D3cold case and whether the platform supports controlling
main power.

It looks like this patch says "we can put devices in D3cold if DT has
'supports-d3'".  But I don't know how to make sense of that because
that requires (a) platform hardware to control main power and (b)
software that knows how to use that hardware.  Wouldn't this require a
little more DT description, like "regulator X controls main power for
this bridge"?  And then an OS would only be able to actually use
D3cold if it knows how to *operate* the regulator, and it doesn't seem
like DT could answer that.

> > If we add annotations that "this device works correctly", we're
> > digging a hole for ourselves because it's impossible to remove those
> > annotations and they complicate all future maintenance.
> > 
> > Bjorn
> 
> -- 
> மணிவண்ணன் சதாசிவம்
  

On Tue, Feb 27, 2024 at 11:37:05AM -0600, Bjorn Helgaas wrote:
> On Tue, Feb 27, 2024 at 10:38:40PM +0530, Manivannan Sadhasivam wrote:
> > On Tue, Feb 27, 2024 at 10:25:35AM -0600, Bjorn Helgaas wrote:
> > 
> > [...]
> > 
> > > > Ok, I got the issue. TBH, I added the device tree property based on
> > > > the existing quirks for the ACPI devices. But none of the DT based
> > > > platforms I'm aware of (even the legacy Qcom MSM8996 chipset
> > > > released in early 2016) doesn't have any issue with D3hot. But I'm
> > > > just nervous to assume it is the case for all the DT based platforms
> > > > in the wild.
> > > > 
> > > > But to proceed further, what is your preference? Should we ammend
> > > > the DT property to make it explicit that the propery only focuses on
> > > > the D3hot capability of the bridge and it works as per the spec
> > > > (PMCSR) or bite the bullet and enable D3hot for all the non-ACPI
> > > > platforms?
> > > > 
> > > > We can add quirks for the bridges later on if we happen to receive
> > > > any bug report.
> > > 
> > > I would assume all devices support D3hot via PMCSR per spec.  We can
> > > add quirks if we discover something that doesn't.
> > 
> > When you say "all devices", are you referring to bridges in DT
> > platforms or the bridges across all platforms?
> 
> This patch is only concerned with DT, so that's what I'm commenting on
> here.  I don't know how to untangle the question of ACPI systems.
> 

Ok, I just wanted to confirm.

> This patch affects platform_pci_bridge_d3(), so just based on the
> "platform" in the function name, I would expect it to be concerned
> with the D3cold case and whether the platform supports controlling
> main power.
> 
> It looks like this patch says "we can put devices in D3cold if DT has
> 'supports-d3'".  But I don't know how to make sense of that because
> that requires (a) platform hardware to control main power and (b)
> software that knows how to use that hardware.  Wouldn't this require a
> little more DT description, like "regulator X controls main power for
> this bridge"?  And then an OS would only be able to actually use
> D3cold if it knows how to *operate* the regulator, and it doesn't seem
> like DT could answer that.
> 

Fair point. And for most of the DT based platforms, there is no dedicated power
supply for the bridge described in DT. So transitioning the bridge to D3cold is
not entirely possible in the OS.

Since we concluded that enabling D3hot for all bridges in DT platforms is the
way to go, I'll drop supporting the DT property in next version.

I'll also remove it from the binding.

- Mani